Rising fear over an outbreak. A new and dangerous strain of the bird flu. So far, contained to china. But tonight, right here in the u.S., Top scientists are now racing to develop a vaccine and if they're concerned, how concerned should the rest of us be? Abc's chief medical editor dr. Richard besser tonight. Reporter: Across asia, travelers are being warned. A new flu has arrived. The target of technicians today in hazmat suits. Birds. Shuts down the poultry markets of shanghai. Officials suspect the new flu, called h-7n9, which normally spreads from bird to bird, is now spreading from birds to people. So far, 16 have fallen ill and six have died. The real fear -- the new flu mutates again and learns to pass from human to human. Because it's brand new, people would have no natural immunity and there's no vaccine to fight it. If one inticketed person with that mutated virus flew from shanghai to los angeles, the virus would easily spread across the u.S. The reality so far? It appears that those who got sick were in direct contact with bird and it is still don tanned just in china. We want to bring in dr. Besser tonight. Came to us from the cdc. How concerned is this team tonight? Reporter: Well, you know, I've been on the phone with cdc. This is what they do best. They are jumping all over this. They are preparing the virus so manufacturers can make a vaccine if that's need. They have developed a test so americans go to china, they can be tested to see if they have this. And they are alerting people, what to watch for. You never know which way one of these outbreaks is going to do.

This transcript has been automatically generated and may not be 100% accurate.

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